a. Field of the Invention
The present invention relates to an objective for video disks and, more particularly, to an objective for video disks with high reduction ratio to be used for recording the modulated signals when making a master disk of photoelectric video disks.
B. Description of the Prior Art
For recording of modulated signals for making the master disk of photoelectric video disks, an objective designed exclusively for said purpose has not yet been known and, therefore, microscope objectives have been generally utilized for the purpose. Due to the fact that a single wavelength in the range of 435.8 nm (g line) to 486.1 nm is used as the recording signal wavelength for recording of modulated signals, it was possible to obtain recording results of somewhat satisfactory level even when such microscope objectives are used. However, microscope objectives have many cemented lens surfaces therein and, moreover, their chromatic aberration is corrected in respect to rays in the range of visible radiation. Therefore, by microscope objectives, it is impossible to record the signals with high accuracy because of the influence of aberrations. Especially when a ray in ultraviolet range (for example, a ray of wavelength 351 nm) is used for the purpose of high-density recording, absorption of light by the binding agent used on said cemented surface occurs in addition to influence of chromatic aberration and, consequently, transparency decreases.
Regarding the performance of objectives for recording, the following properties are especially required: (1) transparancy should be high for rays up to the near ultraviolet radiation or up to the ultraviolet radiation, (2) the resolving power should be high, (3) influence of deffracted rays other than the zero order should be minimized, and (4) contrast of the image should not decrease.
To satisfy the requirement (1) out of the above, it is necessary to select a proper glass material and to prevent absorption of light by the binding agent on the cemented surface. As for the requirement (2), it is necessary to make the numerical aperture as large as possible. As for requirements (3) and (4), it is necessary to correct aberrations favourably and to minimize spherical aberration and coma especially. As for the requirement (4), it is further required to prevent flare. When the objective has a cemented surface, condition of cementing should be taken into consideration in addition to transparency, flare, etc. Because, if the cemented condition of the cemented surface is not good, noise corresponding to the cemented condition will be recorded together with the modulated waves. Moreover, as the incident light is converged into a fine beam, energy of light which passes through a unit area of respective lens surfaces is extremely large and, consequently, disconnection of cemented surface may be sometimes caused.
A master disk for recording is made of a base disk of metal, glass, etc. coated with a light-sensitive agent such as "Photo-resist". The surface of said light-sensitive agent is exposed to the modulated light which passed through and converged by the objective. In case of modulated laser beam, the beam which reaches the surface of light-sensitive surface is converged to a diameter of 1 .mu. or less. Therefore, temperature of the portion exposed to the beam becomes very high and the light-sensitive agent will be evaporated. If the evaporated matter sticks to the objective, contrast of image and resolving power of the objective will decrease.